Mergers lighting the early Universe: enhanced star formation, AGN triggering, and Ly$\alpha$ emission in close pairs at $z=3-9$

Abstract

Galaxy mergers and interactions are often invoked to explain enhanced star formation, black hole growth, and mass build-up of galaxies at later cosmic times, but their effect is poorly understood at high redshift ($z>2$). We use JADES data to analyse a mass-complete sample of 2095 galaxies at $z=3-9$ with $\log (M_{\star }/{\mathrm{M}}_{\odot })=[8,10]$, identifying major merger pairs (projected separation of $5-100$ pkpc, mass ratio $\ge 1/4$) using a probabilistic method. To look for signatures of enhancement in multiple physical properties, we carefully build a control sample of non-pairs that are simultaneously matched in redshift, stellar mass, isolation, and environment to the pair sample. We find a moderate enhancement in specific star formation rate (sSFR) of $1.12\pm 0.05$ at separations $\lesssim 20$ kpc, which is weakly detectable out to $\sim 50$ kpc. We find that at longer averaging timescales (50-100 Myr) the sSFR is more affected by interactions and environment, whereas at shorter timescales (5-10 Myr) it is dominated by internal feedback and burstiness. By averaging star formation histories, we find two distinct populations: pre-first passage/coalescence (monotonically rising SFR) and post-pericentre pairs (earlier peak in SFR). Finally, we find no significant excess of AGN in pairs, suggesting galaxy interactions are not effectively triggering black hole activity at separations $>5$ kpc. Similarly, we also do not detect an excess in the fraction of Lyman-$\alpha$ emitters in pairs, implying that at the probed separations, galaxy interactions are not efficient at enhancing Lyman-$\alpha$ photon production and escape, which may only become important at the smallest scales.

17 pages, 11 figures, 1 table. Submitted to MNRAS. Comments are welcome